ARM: uniphier: remove SPL support for ARMv8 SoCs

It has been a while since ARM Trusted Firmware supported UniPhier SoC
family.  U-Boot SPL was intended as a temporary loader that runs in
secure world.  It is a maintenance headache to support two different
boot mechanisms.  Secure firmware is realm of ARM Trusted Firmware
and now U-Boot only serves as a non-secure boot loader for UniPhier
ARMv8 SoCs.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>